1. Field of the Invention
This invention relates to an indirectly heated filamentary cathode, and more particularly to an indirectly heated filamentary cathode used in a fluorescent luminous device adapted to impinge electron emitted from a cathode on a phosphor-coated anode for luminescence, such as, for example, a fluorescent display tube, a fluorescent luminous tube, a flat type fluorescent luminous device driven at a high voltage, or the like.
2. Description of the Prior Art
A fluorescent luminous device which has been conventionally used in the art generally includes an envelope of a box-like shape constituted by a substrate made of a glass plate, side plates vertically arranged on a periphery of the substrate and a front cover arranged opposite to the substrate through the side plates which are sealedly bonded together by a sealing material. The so-formed envelope is then evacuated to a high vacuum, in which electrodes such as anodes, control electrodes, filamentary cathodes are arranged.
The anodes comprises an anode conductor provided on an inner surface of the substrate and phosphor layers deposited on a surface of the anode conductor.
The control electrodes are disposed above the anodes as required, and the filamentary cathodes are stretchedly arranged the control electrodes.
In the conventional fluorescent luminous device thus constructed, when the filamentary cathodes are heated, electrons or thermions are emitted from the filamentary cathodes. Then, the control electrodes accelerate the electrons and control passing of the electrons therethrough, so that the electrons may be impinged on the phosphor layers of the anodes, resulting in the phosphor layers emitting light.
The filamentary cathode functioning as an electron source of the fluorescent luminous device is generally divided into two types. One is a directly heated filamentary cathode comprising a heating core wire having an oxide material coated thereon so as to serve as an electron emitting material and the other is an indirectly heated filamentary cathode comprising a heating core wire having an electrically insulating layer coated thereon and a conductive cathode substrate arranged on a surface of the electrically insulating layer and having an electron emitting layer made of an electron emitting material and provided thereon.
A conventional indirectly heated filamentary cathode which is considered to be pertinent to the present invention is disclosed in Japanese Patent Application Laying-Open Publication No. 206737/1987 (Japanese Patent Application No. 48274/1986) which was filed by the assignee.
The conventional indirectly heated filamentary cathode disclosed is generally constructed in such a manner as shown in FIGS. 4 and 5. More particularly, it includes a heating core wire 1 made of tungsten into an outer diameter as small as about 5 to 50 .mu.m. On the core wire 1 is coated a heat-resistant electrically-insulating layer 2, which is formed of aluminum oxide into a thickness of about 1 to 50 .mu.m by dipping or electrodeposition.
On the electrically insulating layer 2 is wound a fine metal wire 4 in a coiled manner so as to function as a cathode substrate for the filamentary cathode. The fine metal wire 4 has an outer diameter of several to several tens .mu.m and winding of the wire 4 causes it to serve to protect and reinforce the electrically insulating layer 2, as well as act as the cathode substrate due to application of cathode voltage thereto. Accordingly, metals used for the metal wire 4 include electrically-conductive and heat-resistant metals such as, for example, nickel, tungsten, tantalum, rhenium, rhenium-tungsten alloy and the like.
On the fine metal wire 4 is coated an electron emitting layer 5 made of an electron emitting material capable of emitting thermions therefrom.
In the conventional indirectly heated cathode constructed as described above, an area of the cathode substrate can be increased by reducing a pitch at which the fine metal wire 4 is wound. However, an increase in the number of windings causes a length of the fine metal wire 4 to be wound to be increased, resulting in a resistance of the metal wire 4 being increased. Arrangement of the indirectly heated filamentary cathode of such an increased length in a fluorescent display device leads to a large potential gradient between both ends of the fine metal wire 4. This causes a gradient to occur in a potential of the cathode with respect to an anode, resulting in generation of a gradient in luminescence of a phosphor layer.
A decrease in length of the fine metal wire 4 by increasing a pitch between windings of the wire 4 causes the electron emitting layer 5 to be deposited on only the fine metal wire 4, resulting in the deposition of the layer 5 on portions of the electrically insulating layer 2 between the windings of the wire 4. This leads to a reduction of the electron emitting layer 5, to thereby decrease discharge of electrons from the layer 5. Such construction fails to uniformly transmit heat from the heating core wire 1 to the thermion emitting material layer 5 to cause the distribution of a temperature on the layer 5 to be non-uniform, so that the layer fails to uniformly emit electrons or thermions therefrom.
Further, the electrically insulating layer 2 is formed by coating particles of aluminum oxide on the heating core wire 1 and sintering them, so that the particles aggregate together. This causes the electrically insulating layer 2 to be porous or a number of pin holes to be formed in the layer 2. Unfortunately, such formation of the pin holes causes the electron emitting material to enter the pin holes, accordingly, arrangement of the filamentary cathode in the fluorescent display device leads to an insulation failure of the electrically insulating layer 2 due to contact between the heating core wire 1 and the electron emitting material.